1. Field of the Invention
The present invention relates to conformally coated capacitors. More particularly, though not exclusively, the present invention relates to an apparatus and method for conformally coating a capacitor using a fluidized bed.
2. Problems in the Art
Capacitors exist in the art which are made from a tantalum slug or pellet. To create a conventional tantalum slug, tantalum powder is pressed and then exposed to a process for forming a polarized capacitor. A typical tantalum slug will have an anode comprised of a wire extending from the slug and a conductive surface for the cathode.
It is desirable to coat the tantalum slugs to create conformally coated capacitors. Conformally coated capacitors are desirable because of their high volumetric efficiency (capacitance per volume). If a very thin conformal coating is placed on a tantalum slug, a volumetric efficiency is achieved which is higher than efficiencies from tantalum capacitors where the anode is protected by molding.
The traditional method of applying a conformal coating of insulating epoxy to capacitors has been by electrostatically charged epoxy powder deposition. Electrostatic coating provides the best total coverage in a thin single pass deposition. However, electrostatic coating requires a powder formulation that is favorable to this process and not necessarily to the end product. Electrostatic coating is a process that requires passage of the capacitors one at a time past the charged powder and subsequent removal of the unwanted powder by wiping powder from the cathode face at the conclusion of the powder application. This is required since it is necessary to keep the cathode end of the capacitor clean so that electrical contact can be made with the cathode.
Another prior art method of conformally coating components is by applying epoxy powder by fluidized bed coating. This method involves heating the component to be coated to a temperature above the melting point of the epoxy powder and submerging the heated components into a cloud of epoxy powder. Components coated using the fluidized bed method have traditionally required a coating over the entire surface of the component. Since the cathode end of the tantalum slug would be coated by the epoxy powder using the fluidized bed process, this process will not work for coating the tantalum slugs unless one part of the epoxy is removed to provide contact; this requires additional labor.
Therefore a need can be seen for efficiently and effectively conformally coating tantalum slugs without also coating the cathode end of the slug.